The present invention relates to fuser apparatus for electrostatographic printing machines and in particular to a method and apparatus for preventing the adverse affects of excess oil application.
In the process of xerography, a light image of an original to be reproduced is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto in one of various ways, for example, as by heat and pressure.
In order to affix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky while simultaneously applying pressure. This action causes the toner to flow to some extent into the fibers or pores of support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an adhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon. In these fusing systems, however, since the toner image is tackified by heat it frequently happens that a part of the image carried on the supporting substrate will be retrained by the heated fuser roller and not penetrate into the substrate surface. The tackified toner may stick to the surface of the fuser roll and offset to a subsequent sheet of support substrate or offset to the pressure roll when there is no sheet passing through a fuser nip resulting in contamination of the pressure roll with subsequent offset of toner from the pressure roll to the image substrate.
To obviate the foregoing toner offset problem it has been common practice to utilize toner release agents such as silicone oil, in particular, polydimethyl silicone oil, which is applied to the fuser roll surface to a thickness of the order of about 1 micron to act as a toner release material. These materials possess a relatively low surface energy and have been found to be materials that are suitable for use in the heated fuser roll environment. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner image carried on the support material. Thus, a low surface energy, easily parted layer is presented to the toners that pass through the fuser nip and thereby prevents toner from adhering to the fuser roll surface.
One method of applying a release agent such as silicone oil utilizes a combination donor/metering roll arrangement wherein the metering roll contacts silicone oil in a sump and conveys a metered amount to the donor roll. The metered layer of oil on the metering roll is transported to the donor roll and subsequently to a heated fuser roll. One such Release Agent Metering (RAM) system for applying silicone oils to a heated fuser roll is illustrated in U.S. Pat. No. 4,214,549. The system disclosed therein comprises a donor roll fabricated from a heat insulative and deformable material, for example, silicone rubber which transfers functional release material from a metering roll contacting a supply of release material contained in a sump to the heated fuser roll. A metering blade is supported in contact with the metering roll for metering the release material onto the metering roll to a thickness such that 1 micro liter of oil is dispersed per copy. This type of RAM system dispenses a fixed amount of release agent material to the fuser roll member.
In U.S. Pat. No. 5,200,786 granted to Fromm et al on Apr. 6, 1993, the donor roll of the '549 patent is replaced with a donor brush. As set forth in the '786 patent, the brush donor structure allows for the application of variable amounts of release agent material depending on the mode of operation. In other words, when color prints are being created a greater quantity of silicone oil is applied to the fuser roll compared to the amount applied when operating in the monochrome black mode.
In a donor brush RAM system, both fuser rolls (heated and backup) will reach an equilibrium with the oil on the metering roll unless the oil is removed. If a machine were required to handle a single size substrate the aforementioned equilibrium would not occur because the paper would continuously remove oil from the fuser roll. However, in machines where different size substrates are used a problem arises due to the application of oil over a length of the fuser roll corresponding to the largest (14 inch) paper size used. Thus, when shorter paper (11 inch) is used for extended run lengths, oil accumulates in an area outside the paper path. Oil accumulation must be prevented, otherwise oil will drip into the machine and/or cause deterioration of the fuser roll material when the outer surface of the fuser roll comprises silicone rubber. When roll deterioration occurs, a product of such deterioration is transferred to the longer paper. This results in the phenomena commonly referred to as red printout. Regardless of oil application rate, in the absence of paper, the fuser roll reaches an equilibrium with the metering roll, in both the "donor roll" or "donor brush" RAM systems. Oil thickness on the metering roll, in a donor roll RAM system, is approximately twice the steady state oil thickness on the fuser roll. Therefore, outside paper path, oil thickness is never more than twice. In a donor brush RAM system, oil thickness on the metering roll could be 10-20 times that of the steady state oil thickness of the fuser roll.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.